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    Retired nurse Sue Collins was just beginning the second length of her local pool when her morning swim suddenly became anything but a pleasure.

    Two months ago Sue, 69, who has never suffered from asthma or any other breathing problem in the past, suddenly found herself hard for breath.

    “I felt as if my throat and oesophagus (食道) were closing up,” says Sue.

    Sue is convinced the problem is related to the indoor swimming baths. “I spend half the year in Turkey and swim every day outside in a pool or the sea there and never have this problem,” she says.

    She may be right, because although a trip to the pool is the perfect exercise for many, the chlorine (氯气) used to keep the water free from germs can lead to problems.

    But in most cases it's not the chlorine that causes problems, but the by-products formed when chlorine interacts with other substances — and this is mostly due to people not showering before they enter the pool.

    “This then poisons the water for them and for others,” says Dr. Hull. “The chlorine interacts with sweat and urine(尿素) on the skin and forms by-products called chloramines that float above the surface as a gaseous solution that can be inhaled in.”

    Chloramines are heavier than air so hang over the water where they are easily breathed in. Some believe they may cause lung disorders. A Swedish study in 2013 examining the health of 146 workers at 46 indoor pools found that 17 per cent had airway trouble at work — but no problems at home.

    As Dr. Hull says: ‘People need to remember that showering isn't just for them. It is for the greater good.'

根据短文内容，选择最佳答案，并将选定答案的字母标号填在题前括号内。

如图，DE∥BC，EC=AD，AE=2cm，AB=7.5cm，求DB的长．

如图，DE∥BC，EC=AD，AE=2cm，AB=7.5cm，求DB的长．

如图，DE∥BC，EC=AD，AE=2cm，AB=7.5cm，求DB的长．